There is a trend for downsizing and enhancement in the performance of mobile information terminals such as PCs (Personal Computers), mobile phones, and PDAs (Personal Digital Assistants), and audio-visual devices such as videorecorders and memory audio players, in which lithium-ion secondary batteries are used.
This has led to the desire for higher capacities in lithium-ion secondary batteries. For achieving high capacity, choices and designs of negative-electrode active materials are under study. As negative-electrode active materials for achieving high capacity, metal lithium is being studied, and aluminum, silicon, tin and the like, which are capable of forming alloys with lithium, are being studied (e.g., Non-Patent Document 1). Among others, silicon has a large theoretical capacity, and there has been a proposal for a lithium-ion secondary battery using silicon as an active material (e.g., Patent Document 1).
However, silicon undergoes large volumetric changes when reacting with lithium ions, thus causing a problem in that repetitive charging and discharging invites a lower current collecting ability, such that sufficient cycle characteristics cannot be obtained.
As a negative-electrode active material for solving this problem, SiOx (0<x<2) has been proposed (e.g., Patent Document 2). SiOx has a high capacity, and exhibits stable cycle characteristics. However, there is a problem in that not all of the lithium that was inserted during initial charging is desorbed, thus resulting in a large level of so-called irreversible capacity. In order to solve this problem, it has been proposed to form SiOx into thin films via vacuum evaporation or sputtering (e.g., Patent Document 3).    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-83594    [Patent Document 2] Japanese Laid-Open Patent Publication No. 6-325765    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2004-349237    [Non-Patent Document 1] Solid State Ionics, 113-115, 57, (1998)